Fluid catalytic cracking (FCC) process is employed in petroleum refineries to convert high-boiling hydrocarbon fractions of crude oil to more valuable products such as Liquefied Petroleum Gas (LPG), Gasoline and Diesel. For this, heavy crude oil is chemically broken down into lighter hydrocarbon fractions having a comparatively smaller chain of carbon atoms with the help of one or more catalysts. These high boiling hydrocarbon fractions are then introduced, in multiple streams, into a reactor section to undergo catalytic cracking. This results in lighter hydrocarbon fractions, which are further sent to a fractional distillation column for recovering aforementioned valuable products.
In the FCC process, to minimize the time involved in catalytic cracking, a liquid hydrocarbon stream is vaporized inside the riser reactor to get completely diffused into the pores of the catalyst(s) used. To facilitate this vaporization process, the liquid hydrocarbon stream or the hydrocarbon feed is first atomized. The atomization process, conducted in an atomizer, also referred to as a ‘feed nozzle assembly’, refers to the breaking of a hydrocarbon feed of a given volume into a number of fine droplets to increase the surface area of the hydrocarbon feed with respect to its own initial volume. Also, the hydrocarbon feed is subjected to high temperature during the atomization process which may alter certain physical properties of the hydrocarbon stream, such as viscosity. This further enhances the atomization of the hydrocarbon stream.
The atomization of the hydrocarbon feed is very critical for contacting the hydrocarbon feed with catalyst particles involved in the catalytic cracking. If the feed is introduced without proper atomization, the contact of the feed droplets and catalyst particles will be poor and the heat transfer from the hot catalyst particle to the feed will be less, resulting in low vaporization of feed. Therefore, the hydrocarbon feed is required to be atomized into fine droplets which are of similar sizes of catalyst particles. This essentially helps to increase the contact of feed with the catalyst particles and the transfer of heat from the catalyst to feed for faster vaporization. Conventional atomizers, however, fall short in completely and efficiently atomizing a heavy hydrocarbon feed that is extremely viscous and having very high surface tension. In-efficient atomization leads to non-uniformity in terms of diameter and velocity of the droplets of the atomized hydrocarbon feed. Moreover, it may take considerable time for such hydrocarbon feeds to vaporize, which, in turn, leads to slow and inadequate absorption of heat by the hydrocarbon droplets inside the riser reactor employed during the FCC process.
U.S. Pat. No. 6,142,457 describes a nozzle for atomizing a hydrocarbon feed that is to be catalytically cracked in a commercial FCC unit. The nozzle as disclosed includes a primary conduit for receiving a liquid stream, i.e., the hydrocarbon feed and a secondary conduit for receiving a dispersion medium. The dispersion medium and liquid stream are combined in a mixing zone that is located between the outlet of the secondary conduit and the outlet of the primary conduit. In the mixing zone, the primary conduit inner surface is gradually tapered so the cross-sectional area of the primary conduit outlet is decreased relative to the cross-sectional area of the primary conduit inlet so as to form a liquid film which is atomized as it exits the primary conduit's outlet.
U.S. Pat. No. 4,434,049 is directed to an oil-water emulsion atomizer which is discharged into up flowing dispersed phase catalyst particles at velocities up to sonic velocities to form a suspension under hydrocarbon conversion conditions. The injection devices disclosed in this patent rely on high fluid velocities and high pressure drops to achieve atomization of the oil into fine droplets.
U.S. Pat. No. 5,037,616 teaches that the dispersion of the feed with vapor may be obtained with the aid of a feed injector featured by a venturi tube. Dimensions characterize the geometry of this device such that the speed of the feed and steam mixture reaches sonic conditions at the venture throat. On its turn, the venturi tube shows a cylindrical internal section and is situated between the converging and diverging sections.
U.S. Pat. No. 5,240,183 discloses an atomizing spray nozzle for mixing and atomizing various liquid and gas combinations includes a central liquid conduit, an annular gas passage disposed concentrically about the liquid conduit, a helical spray member, and a spray head. The spray head provides a chamber for the mixing of the gas and liquid and for the discharge of the resulting mixture through the orifice of the head to form an atomized spray. The most common method to produce such a spray pattern is to dispose an elliptical or rectangular orifice at the tip or discharge end of the spray head.
WO2012/041782 teaches the feed nozzle assembly for co-currently introducing gas and liquid into a reactor vessel which feed nozzle assembly comprises (a) an inner tube defining a gas conduit and an outer tube arranged around the inner tube, wherein the outer surface of the inner tube and the inner surface of the outer tube define an annular liquid conduit, and wherein each of the tubes have an inlet end and an opposite outlet end; (b) a first nozzle attached to the outlet end of the inner tube; (c) a second nozzle attached to the outlet end of the outer tube and arranged downstream of the first nozzle, wherein the inner tube contains purging orifices.
Typically, the feedstock used in FCC process is the vacuum gas oil which has a boiling point above 370° C. and CCR (Conradson Carbon Residue) in the range of 0.5 wt. %. In order to derive maximum economic benefit, nowadays, residue feedstocks with higher CCR are also being processed in FCC. If the hydrocarbon feed is a heavy residue such as vacuum residue with viscosity of more than 200 cSt @ 100° C., the atomization becomes more difficult due to its higher viscosity & surface tension, which is one of the major drawbacks of the prior arts.
In view of the above-stated drawbacks, there is a need for an apparatus and a process capable of atomizing a high viscosity hydrocarbon feedstock such as a vacuum residue.